Genetics of Gender Dimorphism in Higher Plants

Abstract

Upon the rediscovery of Mendel’s laws, biologists became very interested in the genetics that distinguished males from females. Many plant systems were examined in the early part of the century for the genetic basis of gender dimorphism. In most cases, herbaceous plants with relatively short generation times were studied for practical reasons and tree species, many of which produce unisexual flowers, were largely ignored. Even though only a sampling of species was studied, the most obvious feature was a lack of uniformity in the genetic mechanisms to distinguish staminate (male) from pistillate (female) flowering plants (previously reviewed in (Dellaporta and Calderon-Urrea 1993; Grant et al. 1994a; Irish and Nelson 1989). Most of the monoecious and dioecious species studied are more closely related to hermaphroditic species that they are to other unisexual species. Only three plant families, the Cucurbitaceae (melons, squash and cucumbers) (Robinson et al. 1976), the Salicaceae (willows) (Westergaard 1958) and the Cannabidaceae (with only three species) (Parker 1990) have predominantly unisexual species. In other families, unisexual species are rare and they are scattered among clades (Yampolsky and Yampolsky 1922). Their phylogenetic distribution indicates that monoecious and dioecious species have evolved independently from hermaphroditic progenitors in many lineages. Therefore, it is not surprising that a variety of genetic mechanisms have emerged to distinguish male and female flower development from hermaphroditic. The only limits to the possible mechanisms are the following demands for reproduction: (1) the development of one type of reproductive organ must be impeded without inhibiting the development of the other (and with minimal change to other floral organs); (2) mating of male and female should result in progeny that are also male and female. Some of the very different genetic mechanisms described are briefly summarized in Table 1. The purpose of this chapter is to illustrate the extent of the variety of genetic mechanisms regulating gender dimorphism in monoecious and dioecious plants.